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You must know and use the formula for pH.
pH = - log [H3O+], where [H3O+] is the molar concentration of hydronium ion.
So, when pH is 8.0 => 8.0 = - log [H3O+] and you can use antilogarithm (the inverse function of logarithm) to find [H3O+], in this way:
[H3O+] = 10^-8 = 1 * 10 ^-8 M
When, pH = 7.0 =>
7.0 = - log [H3O+] => [H3O+] = 1 * 10^ -7 M
Answer: 1*10^-7 mole / liter
